1. Field of the Invention
The present invention relates to a planographic printing plate precursor having sensitivity in the infrared wavelength range, and more specifically, to a negative type planographic printing plate precursor which can be used in so-called direct plate making and is capable of directly making a printing plate using an infrared laser based on digital signals outputted from a computer or the like.
2. Description of the Related Art
There have been remarkable developments in lasers in recent years. In particular, solid lasers or semiconductor lasers (hereinafter, sometimes referred to as “infrared lasers”) emitting infrared rays having wavelengths of 760 nm to 1200 nm, and having a compact size and high output are now readily available. These infrared lasers are very useful as a recording light source when direct plate making is carried out with digital data outputted from a computer or the like. Therefore, recent years have seen an increasing desire for an image recording material having high sensitivity to the above-described infrared ray recording light sources, namely, an image recording material whose solubility in a developing solution changes significantly upon irradiation with infrared rays.
As an example of the above-mentioned negative type image recording material capable of recording images by the use of the infrared lasers, U.S. Pat. No. 5,340,699 discloses a recording material comprising an infrared ray absorbing agent, an acid generating agent, a resol resin, and a novolak resin. However, the above type of negative type recording material needs to be heat-treated after being exposed with lasers to form images. Therefore, negative type image recording materials, which do not require the heat treatment after exposure, are desired.
Further, in a method in which a high-power density exposure using a high-power laser is employed, the exposure areas are intensively and briefly irradiated so as to accumulate with a large amount of light energy during an exposure time, and the light energy is efficiently converted into a heat energy. Due to the heat derived from the light energy, thermal changes such as a chemical change, a phase change, a formation change, or a structural change are caused, and these changes are used to image record. However, in photosensitive layers of conventional planographic printing plate precursors, if an amount to be added of an infrared ray absorbing agent increased to improve recording sensitivity, laser exposing devices and light sources might be contaminated by materials scattered by ablation of the photosensitive layers.
As an example of a negative type image forming material for accomplishing two objectives of improving an image formability and suppressing the ablation, International Publication (WO) No. 97/00777 discloses a negative type image forming material having a photosensitive layer structured by two layers. In this structure, a surface layer, which is a surface being exposed, has photosensitivity so a problem of ablation being easily generated at the surface layer occurs. Further, a post-exposure is required to obtain a rigid image. In view of the suppression of the ablation, a method, in which an overcoat layer, which comprises a water-soluble polymer and can be removed at the time of development, is provided, may also be employed. The aforementioned overcoat layer can easily be removed together with the photosensitive layer in non-image portions. However, there is a concern that, in a case in which the overcoat layer remains on image portions, a large amount of paper loss caused by insufficient ink application will occur.
The object of the present invention is to provide a negative type planographic printing plate precursor which: can be used for direct plate making by being recorded with digital data from a computer, or the like, using a solid laser or a semiconductor laser emitting infrared rays; has high sensitivity to an infrared laser; suppresses ablation of the photosensitive layer during recording; and in addition, is to suppress paper loss, which is caused by the insufficient ink application during printing.